1. Field of the Invention
The present invention relates to a tire and, more particularly, to a tire improved in the wet performances.
2. Description of the Related Art
The tire is provided in its tread with a plurality of grooves for achieving the wet performances.
In order to improve the tire performances on a wet road surface, such as the anti-hydroplaning performances or the wet braking performances, it is necessary to improve the drainage of the grooves.
For improving the drainage, it is necessary to reduce the resistance experienced between the fluid flowing in the grooves and the groove wall faces.
Merely by roughing the groove wall faces, the surface area of the groove walls is increased to raise the resistance.
As the tires having the rough groove wall faces, there have been proposed: (a) Japanese Utility Model Laid-Open No. 037708/1995; (b) Japanese Patent Laid-Open No. 201606/1992; (c) Japanese Patent Laid-Open No. 57704/1991; (d) Japanese Patent Laid-Open No. 009009/1989; (e) Japanese Patent Laid-Open No. 56205/1989; (f) Japanese Patent Laid-Open No. 16617/1993; (g) Japanese Patent Laid-Open No. 151912/1999; (h) Japanese Patent No. 2865765; and (i) Japanese Patent Laid-Open No. 607/1993, for example.
However, the various experiments and investigations of the above-specified proposals have revealed: that the proposals (a), (b), (c) and (g) were encountered by the problem of a raised resistance, that the proposals (d) and (e) were adversely affected by the turbulences in the flow by the sipes in the grooves thereby to increase the resistance; and that the proposal (f) had too large projections to control the flow and increased the groove surface areas to raise the resistance.
In the proposal (h), on the other hand, the groove wall is provided with a plurality of small steps having a height X (i.e., a size measured along the radial direction of the tire) of 0.5 to 2 mm and a width Y (i.e., a size measured along the tread surface of the tire) of 0.2 to 1 mm, as shown in FIG. 26, thereby to improve the performance on the snow.
If these steps of the proposal (h) are assumed as smaller grooves so that their sizes are converted into a depth D and a pitch P of the smaller grooves, these smaller grooves have the depth D of about 0.18 to 0.89 mm and the pitch P of about 0.54 to 2.24 mm. In this case, the improvement in the anti-hydroplaning performances is rather disturbed by the excessively large pitch P of the smaller grooves.
On the other hand, the proposal (i) is also provided with small steps or smaller grooves which are formed against the noises, and these smaller grooves have too large a size as in the proposal (h) to improve the anti-hydroplaning performances.